In machining due to a machine tool, much cutting fluid is supplied into a machining point to cool and lubricate a workpiece or a cutting tool, or to remove cutting chips. In this case, there happen many problems such as environmental pollution due to cutting fluid, a bad influence to human health, a big cost for waste oil disposal of the cutting fluid, shortening span of the cutting tool by supercooling the workpiece, and sliding wear of the cutting tool due to excessive cutting fluid during fine cutting machining. In addition, because much cutting fluid adheres to the cutting chips during machining, the adhered cutting fluid must be removed from the cutting chips in treating or recycling.
To settle these problems, recently, a machine tool that carries out so-called dry cutting has appeared. The dry cutting is what cuts supplying a very small quantity atomized cutting fluid to the machining point.
The applicant has already put a machine tool for carrying out the dry cutting in practice. As shown in FIG. 11, a spindle device of the machine tool is a tool holder 4, to which a cutting tool 13 is fixed, the tool holder being fixed on a spindle 1 through a taper-shank part 5b. 
In this case, a straight holder side atomized cutting fluid passage e2 having a single-shape passage section is provided to the tool holder 4. The cutting fluid passage e2 comprises a central hole “d” of a pull-stud 6, an inner hole “f” of a holder inside connecting pipe 8 and a cutting tool inside passage “g” provided to the thickness of the cutting tool 13. Besides, a spindle side atomized cutting fluid passage e1 comprising an inner hole of an inner pipe 3 is provided to the spindle 1. Here, the inner pipe 3 is provided to a rotating center of the spindle 1 and has a single-shape passage section.
While the cutting tool is machining, atomized cutting fluid generated by an atomized cutting fluid generator provided near the spindle 1 is supplied to the base of the cutting fluid passage e1. Then, the atomized cutting fluid passes through the cutting fluid passage e1, thereafter being ejected from the tip of the tool holder 13 through the cutting fluid passage e2.
In the above-mentioned conventional machine tool, the atomized cutting fluid flows inside the cutting fluid passages e1, e2, each having a single-shape passage cross section, toward the tip of the cutting tool 13. Therefore, as shown in FIG. 10A, the velocity vector “i” of flowing atomized cutting fluid, which expresses the velocity of a microelement thereof in each position in the passage section, is the maximum at a passage center and becomes small towards a passage peripheral wall, defining a parabola “m” due to viscosity or friction with a passage wall. The velocity of atomized cutting fluid on the passage peripheral wall surface becomes theoretically zero.
Here, an important thing is that the velocity of atomized cutting fluid in a comparatively thick layer near the passage peripheral surface is fairly small compared with that of the passage center. Therefore, the atomized cutting fluid is liquefied, thereby easily staying on the peripheral wall surface, preventing stable supply of atomized cutting fluid, causing a waste thereof, and besides, reducing responsiveness in changing over a supply or stop instruction thereof.
The present invention aims to provide a spindle device of a machine tool that can cope with the above-mentioned troubles.